Four-momentum subtraction, introduced in [1]}, removes the thermal momenta exactly from the jet’s four-momentum. The thermal momenta to be subtracted, those that in the absence of the jet would have ended up within the jet's reconstruction region, are determined by adding to the final state particles list a set of `dummy' neutral particles with very small energy and momenta and pointing in the direction of the thermal momentaIn the original version of the four-momentum subtraction, the dummy particles were massless and had the same azimuthal angle and pseudo-rapidity as the thermal momentum. Since Jewel- 2.3.0 the dummies are massive and have the same azimuthal angle, pseudo-rapidity, and rapidity as the thermal momentum. This is preferable since the distance measure in the \(k_\perp \) -family of jet clustering algorithms is based on rapidity separation. The only observable where this change has any visible consequences is the jet mass, which is particularly sensitive to soft particles at the edge of the jet.. In operational terms, these `dummy' particles are the same as the ghosts used by FastJet [2]} during its clustering to determine the jet area. They can get clustered into jets without affecting the jet’s momentum or structure. Thermal momenta, that are matched to a dummy (in the azimuthal angle - rapidity plane) inside a jet, are subtracted from the jet’s momentum. The resulting four vector constitutes the subtracted jet momentum. The procedure was implemented as follows:
